Articles | Volume 21, issue 5
https://doi.org/10.5194/os-21-2149-2025
© Author(s) 2025. This work is distributed under
the Creative Commons Attribution 4.0 License.Assessment of ocean bottom pressure variations in CMIP6 HighResMIP simulations
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- Final revised paper (published on 01 Oct 2025)
- Supplement to the final revised paper
- Preprint (discussion started on 26 Feb 2025)
- Supplement to the preprint
Interactive discussion
Status: closed
Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor
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RC1: 'Comment on egusphere-2025-775', Christopher Piecuch, 03 Apr 2025
- AC1: 'Reply on RC1', Le Liu, 28 May 2025
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RC2: 'Comment on egusphere-2025-775', Anonymous Referee #2, 01 May 2025
- AC2: 'Reply on RC2', Le Liu, 28 May 2025
Peer review completion
AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload
AR by Le Liu on behalf of the Authors (03 Jul 2025)
Author's response
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ED: Publish subject to technical corrections (08 Jul 2025) by John M. Huthnance

AR by Le Liu on behalf of the Authors (09 Jul 2025)
Manuscript
A review of "Assessment of Ocean Bottom Pressure Variations in CMIP6 HighResMIP Simulations" by Liu, Schindelegger, Börger, Foth, and Gou
The authors compare ocean bottom pressure (OBP) variability from GRACE/GRACE-FO retrievals, bottom pressure recorder (BPR) observations, and CMIP6 HighResMIP Simulations for past and future periods. They identify regions where observed and simulated OBP time series agree or not, and they also highlight where simulated OBP variability changes substantially from the present to the future, offerring interpretations in terms of physical oceanographic processes or observational considerations. The analysis largely focuses on periods from a couple months to a decade, and the authors perform their calculations both with and without the mean seasonal cycle removed.
This is a nice paper that presents a solid incremental advance in the science. As the authors explain, changes in OBP can arise from a variety of oceanographic and geodetic processes, so a study like this will be of interest to a wide community of geoscientists. What's more, the paper is well written, the methods and approaches are generally sound, and most scientific inferences are reasonable and justified. The paper should be published after minor revisions to address a few places where the reasoning could be clarified or the analyses could be expanded to make a stronger study. I thank the authors for making a satisfying study on a topic that's largely been overlooked in papers analyzing past and future climate model simulations.
Good luck,
Chris Piecuch, Woods Hole
General comments
Section 2.2 on calculation of OBP anomalies. Why don't the authors use the standard CMIP diagnostic output for OBP (pbo), which is readily available? I'd recommend them to use the proper model diagnostic output because, as the authors explain, right now they're making various assumptions in their calculation of OBP. The errors they're incurring from these assumptions are unclear. First, they're computing density from monthly temperature and salinity using the McDougall and Barker (2011) equation of state. While this sounds reasonable, it risks using an equation of state that's potentially distinct from what the various models use on line. It's also complicated by the nonlinearity of the equation of state (the monthly average of a density time series computed from instantaneous temperatures and salinities is not the same as the density computed from monthly averages of temperature and salinity time series). Second, the authors base their bathymetry on ETOPO1. Again, this sounds reasonable, but model bathymetry can be modified relative to something like ETOPO. Using standard model output alleviates these issues.
Section 2.3 on downscaling GRACE. I'd like the authors to discuss why they downscale GRACE/GRACE-FO data rather than coarsen CMIP6 model output. The downscaling is based on machine learning algorithms that incorporate eddy-permitting ocean circulation models. It's unclear what the associated uncertainties are. Are the downscaled datasets also based on the NEMO modeling framework? If so, then we have the potential of correlated errors between the downscaled GRACE/GRACE-FO products and the CMIP6 models. I'm not recommending the authors fundamentally change their approach. But I would like them to (1.) justify their decision to downscale rather than coarsen and (2.) discuss the associated uncertainties, biases, and other potential implications.
Equation 2 (the variance ratio). The authors define this as the modeled variance divided by the observed variance. When they show results on a linear vertical scale, this definition will tend to visually overemphasize values >1 and de-emphasize values <1 (i.e., the former will span a greater color range than the latter). Therefore, I'd suggest, whenever they're showing R values, the authors use more of logarithmic color scale or vertical axis. That way, values >1 and <1 would visually communicate comparable emphasis.
In all the figures, the authors compare root mean square (RMS) variability from the models and the observations. Typically, modeled RMS values are computed for a fixed time period (e.g., 1980-2014). However, Figure 5 shows the very interesting and important result that, even under a control simulation with presumably stationary statistics, there can be large apparent changes in RMS amplitues (see left column). Because of this stochastic variability, it's unclear whether any of the R values in the preceding figures are significant or not. Therefore, I'd like the authors to perform a more comprehensive error analysis. Rather than computing modeled RMS values over a single period (e.g., 1980-2014), I'd suggest the authors instead compute RMS values for overlapping but separate periods to approximate a distribution of RMS values that would better quantify uncertainty and permit them to test whether simulated OBP variability really is distinct from what we're seeing in the observations.
Starting on line 228, the authors note that models show stronger OBP variability on shelves compared to observations. They may mention that this could arise partly if the models aren't frictional enough or are too shallow (e.g., you expect barotropic ocean response to scale with the inverse of both friction coefficient and ocean depth).
In section 3.5, the authors argue that increased future OBP variability could be related to changes in zonally averaged wind speeds (Figure 6). To me, this is an apples to oranges comparison. Assuming a linear barotropic adjustment such that OBP responds to winds on the same timescale, the more relevant comparison to make here would be to compare RMS (not mean) zonal wind speeds between the two periods. That is, the authors should quantify if the winds will grow more variable in the future (not just if they grow stronger overall).
Line edits
Line 34: Suggest to delete "mainly because they are arbitrary in time"
Line 50: Suggest to delete "by us"
Line 51: Suggest to change "Given the monthly sampling of the data and the fact that model drift precludes the study of trends" to "Given the monthly sampling of the data and the fact that models drift, we are precluded from studying trends"
Line 68: Suggest to change "Ad hoc short names" to "Abbreviations"
Equation 1: Change "\int_{0}^{\eta} \rho_0 g dz" to "\int_{0}^{\eta} \rho g dz" and change the second equal sign to an approximation symbol (since the authors make the very reasonable approximation that density is constant over the vertical distance between 0 and sea level)
Line 97: Change "sketchy" to "uncertain"
Lines 173: Specify *planetary* potential vorticity
Line 184: Suggest to change "baroclinic instability" simply to "instability" to be more general
Line 208: Suggest to change "are therefore likely" to "may be"
Line 262: The concept of geostrophic modes (resonances) is fairly specific (and esoteric; see Greenspan 1968). Suggest to change "geostrophic modes" to simply "variability".
Line 274: Suggest to change "from this behavior" to "to this behavior"
Line 275: pb should be italicized